Recently, ionomer resin has been widely used for cover material of golf balls. This is because the ionomer resin is superior in rebound characteristics, durability, processability and the like. However, since the ionomer resin has high rigidity and hardness, there are problems that in the resulting golf ball, shot feel is hard and poor; and spin performance is not sufficiently obtained, which degrades controllability, when using ionomer resin alone as the cover material.
In order to solve the problems, it is attempted to soften the ionomer resin by various means. For example, it is proposed to blend the hard ionomer resin having high rigidity with terpolymer-based soft ionomer resin (Japanese Patent Kokai Publications No. 3931/1993, Japanese Patent No. 2709950 and the like) or blend the hard ionomer resin with thermoplastic elastomer (Japanese Patent Kokai Publications Nos. 299052/1994, 327794/1994 and the like) to soften the cover.
However, when good shot feel and spin performance are accomplished in case of using the blend of the hard ionomer with the soft ionomer resin, rebound characteristics of the resulting golf bail are largely degraded. When the blend of the hard ionomer with the thermoplastic elastomer is used, rebound characteristics are excellent compared with the blend with the soft ionomer resin, but the compatibility between the both is poor, and durability is poor compared with the blend with the soft ionomer resin.
In order to solve the problems, it has been suggested to improve flexibility, rebound characteristics of golf ball and compatibility of cover material by using epoxidized diene block copolymer in addition to thermoplastic elastomer and ionomer resin as a cover material of golf ball (Japanese Patent Kokai publication No. 176429/1997 and the like). In Japanese Patent Kokai publication No. 176429/1997, thermoplastic resin composition comprising                (A) from 20 to 90 parts by weight of thermoplastic elastomer selected from the group consisting of polyester-based thermoplastic elastomer and polyamide-based thermoplastic elastomer,        (B) from 80 to 10 parts by weight of ethylene-based copolymer selected from the group consisting of ionomer of ethylene copolymer having unsaturated carboxylic acid or its anhydride, and ionomer thereof, and        (C) from 1 to 30 parts by weight of epoxidized diene block copolymer, with a proviso that the sum of (A) and (B) is 100 parts by weightand golf ball produced from the thermoplastic resin composition are disclosed. Since the component (C) is epoxidized, the compatibility is improved compared with the blend of the ionomer resin with the thermoplastic elastomer, but the compatibility is not sufficiently obtained. Therefore, there is problem that laminar separation resistance of the resulting golf ball is poor.        